1. Field of the Invention
The present invention relates to a sintered rare-earth magnet and a method for producing such a magnet.
2. Description of the Related Art
A rare-earth-iron-boron based sintered rare-earth magnet, which is a typical high-performance permanent magnet, has a structure including an R2Fe14B-type crystalline phase (main phase), which is a tetragonal compound, and grain boundary phases, and achieves excellent magnetic properties. In R2Fe14B, R is at least one element selected from the group consisting of the rare-earth elements and yttrium and includes Nd and/or Pr as its main ingredients, Fe is iron, B is boron, and these elements may be partially replaced with other elements. The grain boundary phases include an R-rich phase including a rare-earth element R at a relatively high concentration and a B-rich phase including boron at a relatively high concentration.
The rare-earth-iron-boron based sintered rare-earth magnet will be referred to herein as an “R-T-B based sintered magnet”, where T is a transition metal element consisting essentially of iron. In the R-T-B based sintered magnet, an R2T14B phase (main phase) is a ferromagnetic phase contributing to magnetization and the R-rich phase on the grain boundary is a low-melting nonmagnetic phase.
An R-T-B based sintered magnet is produced by compressing and compacting a fine powder (with a mean particle size of several μm) of a (mother) alloy to make an R-T-B based sintered magnet using a press machine and then sintering the resultant green compact. The sintered compact is then subjected to an aging treatment if necessary. The mother alloy to make such an R-T-B based sintered magnet is preferably made by an ingot process using die casting or by a strip casting process in which a molten alloy is quenched using a chill roller.
To produce an R-T-B based sintered magnet with high coercivity, it is proposed that Nd or Pr, which is used extensively as a rare-earth element R, be partially replaced with a heavy rare-earth element such as Dy or Tb (see Patent Document No. 1, for example). Since Dy and Tb are rare-earth elements with a highly anisotropic magnetic field, the coercivity can be increased effectively by replacing Nd with at least one of those elements at the site of the rare-earth element R in the main phase.
On the other hand, ever since the R-T-B based sintered magnet was developed, a very small amount of Al or Cu has been added to improve the coercivity (see Patent Document No. 2, for example). More specifically, when the R-T-B based sintered magnet was developed for the first time, Al and Cu were regarded as impurities that were inevitably contained in the material alloy. However, it was discovered afterward that Al and Cu are actually almost essential elements that should be added to increase the coercivity of the R-T-B based sintered magnet. It is also known that if Al and Cu were eliminated intentionally, the coercivity of the R-T-B based sintered magnet would be too low to actually use it in various applications.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 60-32306    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 5-234733
The greater the amount of Dy, Tb or Ho added, the higher the coercivity can be. However, Dy, Tb and Ho are very rare elements. That is why if demands for highly refractory magnets to be used in motors for electric cars continue to grow as electric cars become increasingly popular in the near future, the Dy resources will soon be almost exhausted. In that case, there will be serious concerns about a potential upsurge of material costs. For that reason, it is an urgent task to develop some technique of reducing the amount of Dy to be used in high-coercivity magnets. Meanwhile, the additives Al and Cu would increase the coercivity but decrease the remanence Br, which is also a problem.